Croup

01Pathophysiology

Croup is fundamentally a disease of subglottic airway edema. Understanding this single concept unlocks every clinical feature tested on exams.

The parainfluenza virus (most commonly type 1) infects the epithelium of the larynx, trachea, and bronchi. The resulting inflammatory response produces mucosal edema and increased mucus secretion. The key anatomic point is that in children, the narrowest portion of the airway is at the cricoid cartilage (the subglottic region), unlike adults whose narrowest point is at the glottis. The pediatric airway is also funnel-shaped rather than cylindrical, which makes it far more vulnerable to obstruction from even mild swelling.

The clinical significance of this anatomy is explained by Poiseuille's Law, which states that airflow resistance is inversely proportional to the fourth power of the radius ($R\propto \frac{1}{r^4}R\; \backslash propto\; \backslash frac\{1\}\{r^4\}$). This means that 1 mm of circumferential edema in a small pediatric airway causes a dramatic increase in resistance and a corresponding reduction in airflow. This is why a toddler develops stridor and respiratory distress from the same virus that would only cause a hoarse voice in an older child or adult.

Hoarseness results from edema of the vocal cords themselves. The characteristic barky, seal-like cough is produced by turbulent airflow through the narrowed subglottic space and inflamed vocal cords. Inspiratory stridor is present because this is an extrathoracic obstruction: during inspiration, negative intrathoracic pressure causes the already narrowed extrathoracic airway to collapse further inward, worsening the obstruction. Symptoms are classically worse at night because of the natural nocturnal increase in cortisol-mediated anti-inflammatory tone being at its lowest, combined with the supine position and cooler ambient air.

02Classification and Clinical Manifestation

Westley Croup Severity Score

The Westley score is the most widely used system for grading severity. It guides management decisions and is built around five clinical parameters.

Severity Classification

A critical teaching point: decreasing stridor in a worsening child is not improvement. It signals that air movement has become so poor that there is insufficient flow to generate the turbulent sound. This is a classic exam trap.

Subtypes of Croup

03Diagnostic Workup

Croup is a clinical diagnosis. In the vast majority of cases, no laboratory or imaging test is required. A child between 6 months and 3 years presenting with a 1 to 2 day history of URI symptoms followed by the acute onset of a barky cough, hoarseness, and inspiratory stridor, particularly worsening at night, meets the diagnostic criteria based on history and physical examination alone.

Radiographs are not routinely necessary but are ordered when the clinical picture is atypical, when epiglottitis or a foreign body is a concern, or when the child does not respond to initial treatment. The AP neck radiograph is the key imaging study. It demonstrates symmetric subglottic narrowing of the tracheal air column, producing the classic steeple sign (also called the "pencil point" sign or "wine bottle" sign). This replaces the normal squared-off "shouldering" of the subglottic airway. The steeple sign is present in roughly 50% of croup cases, so its absence does not exclude the diagnosis.

The lateral neck radiograph is primarily useful for ruling out epiglottitis. In croup, the lateral view shows a normal, thin epiglottis and distended hypopharynx. In epiglottitis, the lateral view reveals a swollen, rounded epiglottis known as the thumbprint sign.

Pulse oximetry should be monitored in moderate to severe presentations. Significant desaturation () is a late finding that indicates impending respiratory failure.

Blood work, including a CBC or blood cultures, is not indicated in typical viral croup. If the child appears toxic with a high fever and is not responding to dexamethasone and epinephrine, consider bacterial tracheitis and obtain appropriate cultures and imaging.

04Management and Treatment

Dexamethasone is the cornerstone of croup management and is given to every child with croup regardless of severity, including mild cases. The standard dose is 0.6 mg/kg as a single dose, given orally if the child can tolerate it, or intramuscularly if oral administration is not feasible. The onset of action is within 2 to 4 hours for the oral route and somewhat faster for IM. The long half-life of dexamethasone (36 to 54 hours) means that a single dose provides sustained benefit, and repeat dosing is almost never necessary. Prednisolone (1 mg/kg) can be used as an alternative, though evidence favors dexamethasone for its longer duration and single-dose convenience.

Nebulized epinephrine is the rescue therapy for moderate to severe croup. Racemic epinephrine (0.5 mL of 2.25% solution diluted in 3 mL of normal saline) or L-epinephrine (0.5 mL/kg of the 1:1000 concentration, maximum 5 mL) are equally effective. Epinephrine works through alpha-adrenergic vasoconstriction of the subglottic mucosal vasculature, rapidly reducing edema. The onset is within 10 to 30 minutes, but the effect is temporary, lasting only 1 to 2 hours. After the drug wears off, rebound swelling can occur. This is why any child who receives nebulized epinephrine must be observed for at least 2 to 4 hours before discharge to ensure symptoms do not return. A child who remains well 2 to 4 hours after the last dose of epinephrine and has received dexamethasone can be safely discharged.

Humidified air or cool mist has been traditionally recommended but has no proven benefit in randomized controlled trials. It should not be relied upon as a therapeutic intervention, though it is commonly encountered in older references.

Heliox (a helium-oxygen mixture) may be used in severe cases to reduce turbulent airflow through the narrowed airway. It is a temporizing measure and does not treat the underlying inflammation.

Intubation is reserved for children with impending respiratory failure who do not respond to medical therapy. The tube selected should be 0.5 to 1.0 mm smaller than the standard age-predicted size to account for subglottic swelling.

Antibiotics are not indicated in viral croup. If bacterial tracheitis is suspected (toxic appearance, high fever, purulent secretions, failure to respond to croup therapy), IV antibiotics covering Staphylococcus aureus (including MRSA) and Streptococcus species should be initiated.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common way students lose points on croup questions is by confusing croup with epiglottitis. Both feature stridor in a young child, but the clinical profiles are very different. The croup child has a barky cough and is non-toxic; the epiglottitis child is drooling, toxic, and unable to swallow. If a vignette describes drooling, tripod positioning, or a "muffled" voice, the answer is epiglottitis, not croup.

The second trap involves the management sequence. Dexamethasone is given to every child with croup, including mild presentations. Students frequently reserve corticosteroids for moderate or severe cases only. This is incorrect. Another common error is discharging a child immediately after epinephrine administration. The mandatory observation period of 2 to 4 hours must be respected due to the risk of rebound symptoms.

A third trap is the disappearing stridor scenario. A vignette will describe a child with worsening croup whose stridor suddenly becomes quiet. Students may interpret this as clinical improvement. In reality, the loss of audible stridor in a child who is becoming lethargic or agitated represents critically reduced airflow and is a sign of impending respiratory failure.

Finally, remember that croup is a clinical diagnosis. The steeple sign on X-ray is a classic teaching point but is not required to make the diagnosis. If a vignette gives you a textbook history of barky cough, hoarseness, and inspiratory stridor in a toddler after a URI, the next step is treatment with dexamethasone, not an X-ray.

The core competency being tested in croup questions is the ability to recognize a classic presentation and act on it decisively, distinguish it from surgical or life-threatening airway emergencies (epiglottitis, foreign body, bacterial tracheitis), and apply the correct treatment hierarchy: corticosteroids for all, epinephrine for distress, and observation before discharge.